This apparatus is a sample collection apparatus intended to be used with an oil field collection system or between a buyer and seller of light hydrocarbons. One typical location is at a custody transfer point where a purchaser gathers oil into a flow line. Typically, this device will be installed downstream from a producing well, or perhaps several wells connected to a gathering line. This apparatus is connected to the transfer line which removes the collected sample after it has been produced. Sample collection and removal is accomplished after collection for a specified interval. For instance, the sample collection apparatus might be operated for one month whereupon it is removed and replaced by a duplicate apparatus which collects sample for the next month. Often, a separator will produce light ends. These lighter molecules vaporize more readily and are often separated to be stripped into butane or propane. Such light ends are often sold separately, for instance, as butane or propane.
An alternate place of installation is on a line between a buyer and seller of lighter hydrocarbons. Briefly, this might arise in the context of selling refined light molecule hydrocarbons delivered through a line between the selling plant and buying plant. The pipeline stream is typically partially refined and sediment has been removed. The heavier molecules have been removed. The flowing liquid is typically almost free of color, tending to be clear, almost as clear as water in small samples. Assume that the sample is as clear as water. Even in this case, the sample-water interface can be seen in the present apparatus on inspection of accumulated sample.
The contract prices involved may place great importance on the assay of the sample. Over a few days while a sample is collected, the sales of flowing hydrocarbons may total in the millions of dollars. For this reason, sample collection must be accurate and the assay of the sample must be accurately done. In many instances, the sample collection apparatus may collect a very large sample in a fixed cylinder. The cylinder is filled to some maximum volume. The sample is mixed thoroughly as described below. The very large sample is then transferred from the very large cylinder to several small cylinders. As an example, the thoroughly mixed sample is split and transferred into three smaller cylinders which might be denoted as transport cylinders. One is for the buyer, another is for the seller, and a third is for a referee or archieve purposes. As an example, the large cylinder may collect many, perhaps as many as 50 liters while the three transport cylinders each receive and store 10 liters. This invention can be scaled up or down to provide all of the sample receiving cylinders.
The sample collection apparatus of this disclosure is intended for use with sample furnished under pressure. Not only is the sample received under pressure, it is stored under pressure. There is risk that the sample may boil off if it is stored at reduced pressure. Consider as an example collection of the sample at 900 psi. If the gathering line is operated at 900 psi, the sample should also be stored at 900 psi. If it is stored only at atmospheric pressure, there is great risk that lighter hydrocarbons will boil off and the heat content of the sample will be changed as a result of this loss. This loss not only deviates the data, but it also makes storage far more difficult, it being much easier to contain a specified volume within a sealed container as a liquid at elevated pressure. In view of these requirements, back pressure is loaded in the storage container to assure that the pressure of the stored product is maintained in the desired range. This back pressure must be overcome to achieve sample storage. The sample volume is expanded to receive more sample until the storage device is adequately filled.
The sample collection apparatus, having been filled, is then removed and carried away to a test laboratory or other facility. It is optimum that the test be conducted on the container at the same elevated pressure. The device is therefore disconnected from the typical field installation and moved by means of a truck or the like to a laboratory facility. At the laboratory facility, the sample storage container is then connected to deliver the sample to test equipment typically including a chromatograph to determine complete hydrocarbon analysis. Typically, the contract price paid for oil is adjusted according to the hydrocarbon analysis of the sample. In the laboratory, one of the tests applied to the sample is to determine the actual volume of B S & W. This refers to basic sediment and water. This is the trash which is collected in the sample. There is a limit on the B S & W. The limit is typically stated as a percentage. This limit is applied to the sample. A typical size for the sample collection apparatus of this disclosure is 500 cubic centimeters volume. A volumetric sample of this size is deemed to be quite representative of a flow subjected to sampling and analysis in accordance with the teachings of this disclosure. Of course, some samples may be as large as 20,000 cc.
The B S & W specification may limit the total content to 3% or less. Above this level, the sample will be rejected. Below this level, the sample will be accepted. The seller may be required to filter and dewater to reduce B S & W to an acceptable level; often, the produced oil is stored in a tank so that cleaning of the oil to reduce B S & W is not easily done. It is expensive and time consuming to clean the oil to reduce B S & W.
The B S & W normally settles to the bottom. This apparatus includes a bottom located sight glass. The sight glass is sized to hold more than the specified maximum quantity of B S & W. Assume for purposes of illustration that the maximum permissible quantity of B S & W is 2%. On a volume of 500 cc, this amounts to 10 cc. A volume of 10 cc of B S & W is the maximum; this apparatus includes a sight glass volume which exceeds 2% so the interface between the B S & W at the bottom and the petroleum thereabove can be observed. The sight glass and its associated chambers are sized so that perhaps 5% of the total volume is located in this chamber. In other words, the B S & W up to the permissible maximum is visable within the sight glass to see the oil-water interface. In lighter molecules, the accumulated sample is usually rather clear, often as clear as water. In some instances, the two liquids separate at an interface which is difficult to see. This device locates that interface at an easily seen location.
This system thereby accomodates B S & W up to the specified permissible maximum. It enables the laboratory personnel to check for B S & W content. The goal of the laboratory test is to determine pricing dependent on the sample. Most laboratories test the sample on a chromatograph. The routine procedure of testing does not determine water content in the sample. Thus, even before the sample is analyzed by peak detection in a chromatograph, the water content can be determined by the present apparatus. The water-hydrocarbon interface is located in the sight glass. This location is converted into a percentage of total sample. Sample data can be corrected with this information.
With the foregoing in view, the present apparatus is summarized as a sample collection apparatus. It accumulates sample to a specified volume against a specified back pressure. Moreover, it includes a bottom located B S & W collection chamber sized to receive a certain portion of B S & W to enable quick observation, enabling laboratory personnel to observe and measure the total B S & W. Further, the B S & W sample collection apparatus includes a bottom located sight gauge. The sight gauge is normally closed with a product piston filling the sight area. The piston can be lowered after the main cylinder has received a sample sufficiently sized to fill the sight glass area. The main piston keeps pressure on the sample to prevent flashing, namely, boil off of the lighter hydrocarbon molecules.